Magnetic resonance imaging (MRI) apparatus and method for reduced eddy current generation in the imaged object

Abstract

An MRI apparatus and method reduces eddy currents generated by changing gradient magnetic fields used to image an object. Positioning image data from a pilot imaging mode is used to set reference imaging conditions for a subsequent actual diagnostic imaging mode used to acquire diagnostic image data. An eddy current stimulus index is calculated for a gradient magnetic field polarity successively (i.e., iteratively) renewed (i.e., changed) by renewal of imaging conditions and phase-encoding direction. Diagnostic image data is generated during an actual diagnostic imaging mode based on a gradient magnetic field polarity and phase-encoding direction that provides the lowest eddy current stimulus index value, thus using an optimum set of imaging conditions.

1. A magnetic resonance imaging (MRI) apparatus configured to generate image data based on magnetic resonance (MR) signals generated from an object positioned in a static magnetic field and a plurality of gradient magnetic fields by irradiating radio frequency (RF) pulses, the MRI apparatus comprising:
 (1) an imaging condition renewal unit configured to iteratively renew each of plural preliminarily set reference MR imaging conditions for the plurality of gradient magnetic fields; 
 (2) an eddy current stimulus index computing unit configured to compute an eddy current stimulus index corresponding to expected eddy current generation for each of the plurality of gradient magnetic fields at each of the renewed reference MR imaging conditions and to determine an appropriate set of MR imaging conditions for the plurality of gradient magnetic fields based on the computed stimulus indices; and 
 (3) a control unit configured to control MR imaging operations on the object based on the determined appropriate MR imaging conditions. 
 
     
     
       2. The MRI apparatus according to  claim 1 , wherein the imaging conditions renewal unit renews a polarity of at least one of (a) a slice-selection gradient magnetic field and (b) a phase-encoding gradient magnetic field of the reference MR imaging conditions for the plurality of gradient magnetic fields. 
     
     
       3. The MRI apparatus according to  claim 1 , wherein the imaging conditions renewal unit renews at least a phase-encoding direction for a prescribed slice-section in the reference MR imaging conditions for the plurality of gradient magnetic fields. 
     
     
       4. The MRI apparatus according to  claim 3 , wherein the imaging conditions renewal unit renews phase-encoding direction within an effective angle range for a parallel imaging (PI) operation. 
     
     
       5. The MRI apparatus according to  claim 1 , wherein the stimulus index calculation unit calculates stimulus indices based on the rate of change of magnetic field intensities for each of the slice-selection gradient magnetic field, the phase-encoding gradient magnetic field and the frequency-encoding gradient magnetic field, or based on a composite rate of change of magnetic field intensity acquired by combining the rate of change of magnetic field intensities occurring at a same switching time of the gradient magnetic field in an MRI pulse sequence. 
     
     
       6. The MRI apparatus according to  claim 1 , wherein the stimulus index calculation unit searches for a set of reference imaging conditions where the maximum stimulus index takes its lowest value at a pulse sequence field switching time calculated for each of plural sets of renewed reference imaging conditions. 
     
     
       7. The MRI apparatus according to  claim 1 , further comprising:
 a reference imaging conditions setting unit; 
 wherein the reference imaging conditions setting unit sets reference imaging conditions of the gradient magnetic fields for acquiring diagnostic image data in an actual diagnostic imaging mode by using object positioning image data acquired in a pilot imaging mode. 
 
     
     
       8. The MRI apparatus according to  claim 1 , wherein the control unit effects MR imaging with a preceding spatial resolution based on spatial resolution of the searched imaging conditions for the gradient magnetic fields. 
     
     
       9. The MRI apparatus according to  claim 1 , wherein the control unit presets time parameters, including at least one of an echo time (TE) and a repetition time (TR) based on the searched imaging conditions for the gradient magnetic fields. 
     
     
       10. The MRI apparatus according to  claim 9 , wherein the control unit effects magnetic resonance imaging with a preceding contrast resolution or time resolution by shortening time parameters, including at least one of echo time (TE) and repetition time (TR). 
     
     
       11. The MRI apparatus according to  claim 1 , wherein the stimulus index calculate unit searches for a set of imaging conditions where the rate of change of magnetic field intensity (dB/dt) takes its lowest value. 
     
     
       12. The MRI apparatus according to  claim 9 , wherein the control unit controls thickness of a slice cross-section or a spatial resolution for reference imaging conditions for the gradient magnetic fields corresponding to the lowest value in a prescribed range of the rate of change of magnetic field intensity (dB/dt) searched by the stimulus index calculation unit. 
     
     
       13. The MRI apparatus according to  claim 9 , wherein the control unit controls the rate of change of magnetic field intensity (dB/dt) searched by the stimulus index calculation unit so that time resolution takes a maximum value. 
     
     
       14. The MRI apparatus according to  claim 9 , further comprising:
 a display unit including a monitor for displaying reference imaging conditions for the plurality of gradient magnetic fields searched by the stimulus index calculation unit so as to permit operator selection of an appropriate set of reference imaging conditions for gradient magnetic fields. 
 
     
     
       15. The MRI apparatus according to  claim 14 , wherein the display unit displays a plurality of phase-encoding direction data for different angles or a plurality of rectangular regions of interest (ROI) in order to permit operator selection of a desired phase-encoding direction data or a desired ROI. 
     
     
       16. The MRI apparatus according to  claim 1 , wherein the image data is acquired by imaging at an oblique cross-section in an operator-selected optional direction. 
     
     
       17. A magnetic resonance (MR) imaging method configured to generate MR image data based on MR signals generated from an object positioned in a static magnetic field and a plurality of switched gradient magnetic fields by irradiating radio frequency (RF) pulses, the MR imaging method comprising:
 (1) renewing preliminarily set reference MR imaging conditions for the plurality of switched gradient magnetic fields; 
 (2) computing an eddy current stimulus index for each of the plurality of switched gradient magnetic fields using each set of the renewed reference MR imaging conditions and determining an appropriate set of MR imaging conditions for the gradient magnetic fields based on the computed eddy current stimulus indices; and 
 (3) controlling an MR imaging operation on the object based on the determined appropriate set of MR imaging conditions. 
 
     
     
       18. A magnetic resonance imaging (MRI) method configured to use an optimized set of reference imaging conditions which reduces undesired eddy current generation in an object being imaged, said method comprising:
 (1) using at least one non-transitory programmed computer configured (a) to calculate a maximum eddy current stimulus index for a designated MRI method and an initially designated set of reference imaging conditions; (b) to iteratively repeat the calculation of a maximum eddy current stimulus index for said designated MRI method and different designated sets of reference imaging conditions; and (c) to designate as an optimum set of reference imaging conditions the set for which the calculated maximum eddy current stimulus index had the smallest value; and 
 (2) using an MRI apparatus (a) to perform a diagnostic MRI data acquisition scan using said designated MRI method and said optimum set of reference imaging conditions; and (b) to process such acquired MRI data into image data which is stored or displayed as a diagnostic image. 
 
     
     
       19. The method according to  claim 18 , wherein the iteratively changed reference imaging conditions include at least one of (a) polarity of a slice-selecting gradient field, (b) polarity of a phase-encoding gradient field, (c) direction of a phase-encoding gradient field, (d) an echo time (TE), and (e) a repetition time (TR). 
     
     
       20. The method according to  claim 18 , wherein said initially designated set of reference imaging conditions is designated by an operator using displayed MRI data acquired in a preliminary pilot imaging mode.